One-electron capture and target-ionization in He-neutral-atom collisions

One-electron capture and target-ionization cross-sections in collisions of He⁺ ions with neutral atoms: He⁺ + A → He + A⁺ and He⁺ + A → He⁺ + A⁺ + e, A = H, He(1s², 1s2s), Ne, Ar, Kr, Xe, are calculated and compared with available experimental data over the broad energy range E = 0.1 keV/u–10 MeV/u of He⁺ ions. The role of the metastable states of neutral helium atoms in such collisions, which are of importance in plasma physics applications, is briefly discussed. The recommended cross-section data for these processes are presented in a closed analytical form (nine-order polynomials) which can be used for a plasma modeling and diagnostics.     

Design of moderator and multiplier systems for D–T neutron source in BNCT

Boron Neutron Capture Therapy (BNCT) is an outstanding way to treat Glioblastoma Multiforme. Epithermal neutrons with energy from 1 eV to 10 keV represent the most effective range for brain tumor therapy. In this research we have focused on ³H(d, n)⁴He reaction as a neutron source using Cock Craft Walton accelerator. High neutron yield with 14.1 MeV energy can be generated via accelerating a deuteron beam with 110 keV energy.A Monte Carlo simulation code (MCNP4C) was used to design the D–T source. Pb and ²³⁸U are suggested as neutron multipliers; AlF₃ and BeO as a moderator and reflector, respectively. An Al layer is used for decreasing the ratio of fast to total neutron fluxes. Epithermal neutron flux in the suggested system is 10⁸ n/cm² s and is a suitable flux for BNCT applications. Finally the suggested configuration is compared to the most recent works and it is shown that the proposed configuration works better.     

Colliding Beam Fusion Reactors

The recirculating power for virtually all types of fusion reactors has previously been calculated [1] with the Fokker–Planck equation. The reactors involve non-Maxwellian plasmas. The calculations are generic in that they do not relate to specific confinement devices. In all cases except for a Tokamak with D–T fuel the recirculating power was found to exceed the fusion power by a large factor. In this paper we criticize the generality claimed for this calculation. The ratio of circulating power to fusion power is calculated for the Colliding Beam Reactor with fuels D–T, D–He³ and p–B¹¹. The results are respectively, 0.070, 0.141 and 0.493.     

On the tenuous nature of passivity and its role in the isolation of HLNW

The use of reactive metals and their alloys (e.g., Ni–Cr–Mo–W–Fe and Fe–Cr–Ni alloys) for isolating high level nuclear waste (HLNW) from the biosphere relies upon a continuing state of kinetic passivity of the metal surface. Without this state, which is due to the formation and continued existence of a ‘passivating’ oxide film, the alloy would react rapidly with components of the ambient environment (oxygen, water) and the structural integrity of the container would be compromised. The stability of the barrier oxide layers of bilayer passive films that form on metal and alloy surfaces, when in contact with oxidizing aqueous environments, is explored within the framework of the point defect model (PDM) using phase-space analysis (PSA), in which the rate of growth of the barrier layer into the metal, (dL⁺/dt), and the barrier layer dissolution rate, (dL⁻/dt), are plotted simultaneously against the barrier layer thickness, assuming that both processes are irreversible. A point of intersection of dL⁻/dt with dL⁺/dt indicates the existence of a metastable barrier layer with a steady state thickness that is greater than zero. If dL⁻/dt > (dL⁺/dt)_L=0, where the latter quantity is the barrier layer growth rate at zero barrier layer thickness, the barrier layer cannot exist, even as a metastable phase, as the resulting thickness would be negative. In any event, phase space analysis of the PDM permits specification of the conditions over which reactive metals will remain passive in contact with aqueous systems and hence of the conditions that must be met for the viable use of reactive metals and alloys for the isolation of HLNW.     

Influence of nitrogen ion implantation energies on surface chemical bonding structure and mechanical properties of nitrogen-implanted silicon carbide ceramics

Nitrogen ions were implanted into silicon carbide ceramics (N⁺-implanted SiC) at different ions energies. The surface chemical bonding structure of N⁺-implanted SiC ceramics were investigated by using X-ray photoelectron spectroscopy (XPS). The hardness of N⁺-implanted SiC ceramics was measured using nano-indenter, and the friction and wear properties of the N⁺-implanted SiC/SiC tribopairs were studied using ball-on-disk type tribo-meter in water lubrication. The wear tracks were observed using non-contact surface profilometer and scanning electron microscope (SEM). The results showed that the surface roughness of N⁺-implanted SiC ceramic was higher than that of SiC ceramic, and some chemical bonds such as Si–N, C–C, CN and C–N bonds were formed in N⁺-implanted layer besides Si–C bonds. In comparison of SiC ceramic’s hardness, the hardness of N⁺-implanted SiC ceramics at 30 and 50 keV was higher while that at 65 keV was lower. Under water lubrication, the friction coefficient and the specific wear rates for the N⁺-implanted SiC/SiC tribopairs were all lower than those of the SiC/SiC tribopairs, and displayed the lowest values at 50 keV. According to XPS analysis, it was concluded that the high wear resistance and low friction coefficient for the N⁺-implanted SiC/SiC tribopairs were attributed to the formation of carbon rich composite on the surface of N⁺-implanted SiC ceramics.     

Ductile–brittle transition of polycrystalline iron and iron–chromium alloys

Fracture toughness of polycrystalline Fe, Fe–3%Cr and Fe–9%Cr was measured by four-point bending of pre-cracked specimens at temperatures between 77 K and 150 K and strain rates between 4.46 × 10⁻⁴ and 2.23 × 10⁻² s⁻¹. For all materials, fracture behaviour changed with increasing temperature from brittle to ductile at a distinct brittle–ductile transition temperature (T_c), which increased with increasing strain rate. At low strain rates, an Arrhenius relation was found between T_c and strain rate in each material. At high strain rates, T_c was at slightly higher values than those expected from extrapolation of the Arrhenius relation from lower strain rates. This shift of T_c was associated with twinning near the crack tip. For each material, use of an Arrhenius relation for tests at strain rates at which specimens showed twinning gave the same activation energy as for the low strain rate tests. The values of activation energy for the brittle–ductile transition of polycrystalline Fe, Fe–3%Cr and Fe–9%Cr were found to be 0.21, 0.15 and 0.10 eV, respectively, indicating that the activation energy for dislocation glide decreases with increasing chromium concentration in iron.     

Numerical Study on Effects of Fuel Mixture Fraction and Li-6 Enrichment on Neutronic Parameters of a Fusion–Fission Hybrid Reactor

This study presents the effects of mixture fractions of nuclear fuels (mixture of fissile–fertile fuels and mixture of two different fertile fuels) and ⁶Li enrichment on the neutronic parameters (the tritium breeding ratio, TBR, the fission rate, FR, the energy multiplication ratio, M, the fissile breeding rate, FBR, the neutron leakage out of blanket, L, and the peak-to-average fission power density ratio, Γ) of a deuterium–tritium (D–T) fusion neutron-driven hybrid blanket. Three different fertile fuels (²³²Th, ²³⁸U and ²⁴⁴Cm), and one fissile fuel (²³⁵U) were selected as the nuclear fuel. Two different coolants (pressurized helium and natural lithium) were used for the nuclear heat transfer out of the fuel zone (FZ). The Boltzmann transport equation was solved numerically for obtaining the neutronic parameters with the help of the neutron transport code XSDRNPM/SCALE4.4a. In addition, these calculations were performed by also using the MCNP4B code. The sub-limits of the mixture fractions and ⁶Li enrichment were determined for the tritium self-sufficiency. The considered hybrid reactor can be operated in a self-sufficiency mode in the cases with the fuel mixtures mixed with a fraction of equal to or greater than these sub-limits. Furthermore, the numerical results show that the fissile fuel breeding and fission potentials of the blankets with the helium coolant are higher than with the lithium coolant.     

Self-organized formation of layered carbon–copper nanocomposite films by ion deposition

The quasi-simultaneous deposition of low energy-mass-selected C⁺ and metal⁺ ions leads to the formation of metal–carbon nanocomposites. In the case of C⁺ and Cu⁺ deposition, a homogeneous distribution of small copper clusters in an amorphous carbon matrix is expected. However, at a certain C⁺/Cu⁺ fluence ratio and energy range, alternately metal-rich and metal-deficient layers in an amorphous carbon matrix with periods in the nm range develop have been observed. The metal-rich layers consist of densely distributed crystalline Cu particles while the metal-deficient layers are amorphous and contain only few and small Cu clusters. The formation of multilayers can be described by an interplay of sputtering, surface segregation, ion induced diffusion, and the stability of small clusters against ion bombardment. This formation has been investigated for the a-C:Cu system with respect to the ion energy and the C⁺/Cu⁺ fluence ratio. The sputter coefficient S_M = r_f S_CCu + S_CuCu is the parameter to switch between layer growth (S_M < 1) and homogeneous cluster distribution (S_M > 1).     

One and two electron transitions in energetic ion and helium collisions

The single ionization (SI) and double ionization (DI), single and double ionization accompanying one electron loss (SS) of the projectile and the single and double electron capture of the projectile from target atom are investigated for 2.0–8.0 MeV Na^q+–He and C1^q+–He collisions. The cross-section ratio of the double ionization (DI) to that of the SI, the ratio of SS+DI to SS+SI, and ratio of double capture (DC) cross-section to that of single capture (SC), are measured by means of time-of-flight technique. The velocity and charge state dependences of the measured cross-section ratios are studied and discussed. The obtained capture cross-section ratios are compared with the theoretical calculations using the classical over-barrier model.     

Peculiarities of defect generation in Si+-implanted GaAs (2 1 1)

Peculiarities of the defect generation during implantation of (2 1 1) GaAs with Si⁺ ions and doses below the amorphisation dose of GaAs have been investigated by X-ray diffraction, the secondary ion mass-spectroscopy (SIMS) and transmission electron microscopy. It was shown, that in such implanted layers less radiation defects will be formed and these defects are more easily annealed by rapid photon annealing (RPA) than in (1 0 0)-oriented wafers.     

Simulation of the Behavior of Activated Products of Corrosion during RBMK-1000 Startup after Scheduled Major Overhaul

A simple model is proposed for calculating the activation of corrosion products during startup and under normal operation conditions of a reactor. The model parameters, such as the rate of removal of radionuclides from the loop and the wash-off and deposition constants of radionuclides in the reactor core, are determined for certain radionuclides.It is shown that the removal rate of activated corrosion products is 3–10 times higher than previously assumed, and is (2.4–6)·10^–4 sec^–1 for ⁵¹Cr and (7.5–10)·10^–4 sec^–1 for ⁶⁰Co. The removal of ⁵¹Cr from a forced multiple circulation loop as a result of the higher solubility of this radionuclide is determined primarily by bypass purification, and the removal of ⁶⁰Co is determined primarily by removal with steam and subsequent purification of the condensate. The model calculations agree satisfactorily with observations.     

Formation of Gas Pores in Nickel Alloys and Structural Steel under Irradiation by Helium Ions

Transmission electron microscopy is used to study the development of helium porosity in binary alloys of nickel with elements possessing a different dimensional atomic mismatch with nickel – from negative (beryllium and silicon) to positive (molybdenum, tungsten, aluminum, titanium, tantalum, tin, and zirconium), in structural steels ChS-68, ÉP-150, and the nickel alloy KhNM. The gas pores were produced by irradiation with 40 keV He⁺ up to fluence 5·10²⁰ m^–2 at 650 and 20°C followed by annealing at 650°C for 1 h. It is shown that under high-temperature annealing beryllium and silicon, relative to nickel, give rise to the formation of larger bubbles, while elements with a larger positive size mismatch with nickel atoms substantially decrease the size and increase the density of the bubbles. On the whole, as atomic radius and the concentration of the alloying element increases in alloys, the gas swelling of the irradiated layer decreases. Under post-irradiation annealing, bubbles with the largest diameter and the lowest density develop in nickel. Any alloying used decreases the size and increases the density of bubbles. The data obtained are discussed from the standpoint of the formation of various vacancy complexes of helium and their thermal stability.     

Damage accumulation in gallium nitride irradiated with various energetic heavy ions

In this work a study of damage production in gallium nitride via elastic collision process (nuclear energy deposition) and inelastic collision process (electronic energy deposition) using various heavy ions is presented. Ordinary low-energy heavy ions (Fe⁺ and Mo⁺ ions of 110 keV), swift heavy ions (²⁰⁸Pb²⁷⁺ ions of 1.1 MeV/u) and slow highly-charged heavy ions (Xeⁿ⁺ ions of 180 keV) were employed in the irradiation. Damage accumulation in the GaN crystal films as a function of ion fluence and temperature was studied with RBS-channeling technique, Raman scattering technique, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).For ordinary low-energy heavy ion irradiation, the temperature dependence of damage production is moderate up to about 413 K resulting in amorphization of the damaged layer. Enhanced dynamic annealing of defects dominates at higher temperatures. Correlation of amorphization with material decomposition and nitrogen bubble formation was found. In the irradiation of swift heavy ions, rapid damage accumulation and efficient erosion of the irradiated layer occur at a rather low value of electronic energy deposition (about 1.3 keV/nm³), which also varies with irradiation temperature. In the irradiation of slow highly-charged heavy ions (SHCI), enhanced amorphization and surface erosion due to potential energy deposition of SHCI was found. It is indicated that damage production in GaN is remarkably more sensitive to electronic energy loss via excitation and ionization than to nuclear energy loss via elastic collisions.     

Damage accumulation and annealing in 6H–SiC irradiated with Si+

Damage accumulation and annealing in 6H-silicon carbide (α-SiC) single crystals have been studied in situ using 2.0 MeV He⁺ RBS in a 〈0 0 0 1〉-axial channeling geometry (RBS/C). The damage was induced by 550 keV Si⁺ ion implantation (30° off normal) at a temperature of −110°C, and the damage recovery was investigated by subsequent isochronal annealing (20 min) over the temperature range from −110°C to 900°C. At ion fluences below 7.5 × 10¹³ Si⁺/cm² (0.04 dpa in the damage peak), only point defects appear to be created. Furthermore, the defects on the Si sublattice can be completely recovered by thermal annealing at room temperature (RT), and recovery of defects on the C sublattice is suggested. At higher fluences, amorphization occurs; however, partial damage recovery at RT is still observed, even at a fluence of 6.6 × 10¹⁴ Si⁺/cm² (0.35 dpa in the damage peak) where a buried amorphous layer is produced. At an ion fluence of 6.0 × 10¹⁵ Si⁺/cm² (−90°C), an amorphous layer is created from the surface to a depth of 0.6 μm. Because of recovery processes at the buried crystalline–amorphous interface, the apparent thickness of this amorphous layer decreases slightly (<10%) with increasing temperature over the range from −90°C to 600°C.     

Behavior of mechanical properties of nickel-alloyed reactor pressure vessel steel under neutron irradiation and post-irradiation annealing

The effect of neutron irradiation and post-irradiation thermal annealing on tensile and impact properties of Cr–Ni–Mo steel used for WWER-1000 reactor pressure vessel (RPV) manufacturing was studied. A gap in yield stress and ultimate tensile stress fluence dependence at the fluence range of 0–3×10²³ neutrons m⁻² was observed while ductile-to-brittle transition temperature (DBTT) was continuously increasing with damage dose. The post-irradiation annealing recovery of tensile properties was found to be higher than the one of impact properties. Over-recovery of tensile properties due to 460 and 490°C post-irradiation annealings were observed. The annealing effectiveness of WWER-440 and WWER-1000 grades was compared. Nickel was supposed to affect both the radiation sensitivity and the post-irradiation residual DBTT shift of WWER-1000 type steel.     

Hydrogen implantation defects in MgO

Deuterium and hydrogen ions with an energy of 15 keV have been implanted in virgin MgO (1 0 0) single crystals and in single crystals containing helium implantation generated microcavities. Doses were varied from 2 × 10¹⁵ to 2 × 10¹⁶ cm⁻². The samples were annealed from room temperature to 950 K. The defects produced by hydrogen and the trapping of hydrogen at the defects were monitored by photon absorption and positron beam analysis. With this novel technique a depth distribution of defects can be determined for implantation depths from 0 to 2000 nm. The technique is very sensitive for vacancy and vacancy clusters, i.e. sites with low electron density. After 950 K annealing microcavities were observed for the 2 × 10¹⁶ cm⁻² dose but not for the 10 times lower dose. During annealing up to 750 K point defects are mobile but the defect clusters remain small and filled with hydrogen. In samples which contain already microcavities, point defects and deuterium from the deuterium irradiation are accumulated by the microcavities.     

Corrosion resistance of stainless chrome-nickel steels in sodium as a function of its oxygen content

The article describes an experimental apparatus for studying the corrosion resistance of construction materials in circulating liquid sodium, as well as methods for the continuous removal of oxides from the sodium by means of porous metal filters and for the determination of its oxygen content. It describes the results of corrosion experiments and measurements of mechanical properties performed on six brands of stainless steel, specimens of which were placed in sodium containing 3·10^–3% and (4–5)·10^–2% oxygen at a temperature of 550°. The flow rate was 1.5 m/sec. The results of the corrosion experiments indicate that the corrosion rate of Type 18CrSNi stainless steels in sodium containing as much as (4–5)·10^–2% oxygen does not exceed 21.3 mg/dm² per month; these steets are not subject to intergranular corrosion, and their mechanical properties remain virtually machanged.G. V. Akimov State Scientific Research Institute for the Preservation of Materials, Prague, Czechoslovak Soviet Socialist Republic Translated from Atomnaya Énergiya, Vol. 14, No. 4, pp. 375–382, April, 1963     

Neutron irradiation effects on superconducting wires and insulating materials

On the progress of the Deuterium–Deuterium (D–D) or Deuterium–Tritium (D–T) burning plasma devices, the importance of neutron irradiation on superconducting magnet materials increases and the data base is desired to design the next generation devices. To carry out the investigations on the effect of neutron irradiation, neutron irradiation fields are required together with post-irradiation test facilities. In these several years, a collaboration network of neutron irradiation effect on superconducting magnet materials has been constructed. 14 MeV neutron irradiation was carried out at Fusion Neutronics Sources (FNS) in Japan Atomic Energy Agency (JAEA) and fission neutron irradiation was performed at JRR-3 in JAEA. After the irradiation, the Nb₃Sn, NbTi and Nb₃Al samples were sent to High Field Laboratory for Superconducting Materials (HFLSM) in Tohoku University and the superconducting properties were evaluated with 28 T hybrid magnet. Also, the organic insulation materials are considered to be weaker than superconducting materials against neutron irradiation and cyanate ester resin composite was fabricated and tested at the fission reactor. One clear result on Nb₃Sn was the property change of Nb₃Sn by 14 MeV neutron irradiation over 13 T. The critical current was increased by 1.4 times around 13 T but the increment of the critical current became almost zero at higher magnetic fields and the critical magnetic field of the irradiated sample showed almost the same as non-irradiated one.     

A simple way to analyze infrared reflectivity spectra of p-type Hg0.78Cd0.22Te implanted in various conditions

Different ion-implanted p-type Hg_0.78Cd_0.22Te samples were analyzed by infrared reflectivity in the 2–20 μm wavelength range. We show how to derive some characteristic values of the free carriers induced by ion implantation from simple models of the implanted samples. For low energy implantations (Al (320 keV)) an excess of electrons with concentration n⁺ ≈ 5 × 10¹⁷ cm⁻³ for doses 10¹² and 10¹⁴ ions cm⁻² is observed between the surface and the projected range R_p of the ions, in agreement with the well-known change of type of the free carriers induced by the ion implantation in this kind of samples. High energy α particle (0.8 and 2 MeV, 10¹⁴ ions cm⁻²) implantations lead to a pronounced inhomogeneous concentration of free electrons with n⁺ ≈ 9.2 × 10¹⁶ cm⁻³ between the surface and R_p where a negligible amount of defects due to the nuclear energy loss is formed, and n⁺ ≈ 1.6 × 10¹⁷ cm⁻³ between R_p and R_p + ΔR_p, ΔR_p being the longitudinal straggling, where the defect production rate through the nuclear energy loss mechanism is maximum.